Printed circuit boards contain through holes which have been drilled and typically plated. Due to drilling and plating limitations, these plated through holes are quite large with respect to the wiring. Through holes are often filled for a variety of reasons; for example, to recapture otherwise lost real-estate, or to prevent liquids used in fabrication steps from entering the through holes.
One method of filling holes involves forcing a fill composition, typically a copper filled epoxy paste, through a mask into the holes of the printed circuit board. However, such a fill composition often bleeds between the mask and the board. Also when the mask is lifted off, paste adheres to the mask and is pulled out of the through holes. Moreover, since a typical mask can only be used once, the mask must be drilled each time a set of holes is to be filled.
Conventional hole fill materials suffer from the further disadvantage in that they are not suitable when subsequent processing steps involve electroless plating of metal, because such fill materials are typically not chemically compatible with the electroless gold plating bath. The gold electroless plating bath tends to degrade the fill material, and/or leaches the fill material into the bath which interferes with the electroless plating.
It would be desirable to have an efficient technique for reliably and completely filling holes that does not cause bleeding, and does not degrade in electroless plating baths.